[{"_id":NumberLong(2761633),"z.z.z":"ok","test":{"a":"a","b":{"b":1,"b.b":"b"},"c":"c"}}]

db.collection.aggregate([{$replaceWith:{$setField:{field:"test",input:"$$ROOT",value:{$unsetField:{field:"b.b",input:{$getField:"test"}}}}}}])